This invention relates generally to a device, such as a governor for preventing overspeed by cutting or breaking an electrical circuit when the rotational speed of a rotary member driven by an electrical motor exceeds a predetermined value, and particularly, the present invention relates to a safety device for various electrically driven tools, such as a disk grinder, an electric planer, an electric circular saw, and the like.
Certain conventional electrical tools, such as a disk grinder, are equipped with a safety device having a cutter which is arranged to protrude or jump up radially with respect to the rotary shaft of the tool when the rotational speed of the rotary shaft exceeds a predetermined speed. The cutter is received in a rotary member, and is normally biased by a force of an elastic member, such as a coil spring, toward the center of the rotary member. The cutter protrudes outward when the centrifugal force applied to the cutter becomes greater than the force of the elastic member. A power feed path is located stationarily outside the rotary member so that the feed path will be broken when the cutter of the rotary member protrudes outward.
The above-mentioned predetermined rotational speed of the rotary shaft or the rotary member, above which the cutter protrudes outward, is determined by the mass of the cutter and a tensile force applied from the elastic member to the cutter in the radial direction of the rotary member. Meanwhile, the predetermined rotational speed has great variations throughout a number of products of safety devices for preventing overspeed due to variations in sizes, weights and characteristics of various parts which constitute the safety device. In order to reduce such undesirable variations throughout a number of products, it is theoretically possible to change the weight of the cutter and/or the characteristic of the elastic member. However, it is very troublesome and time-consuming to perform such an adjustment after the safety device has been assembled. Furthermore, when it is intended to change the predetermined rotational speed above which the cutter protrudes outward, the cutter has to be replaced with another having different weight or the elastic member has to be replaced with another having different characteristics.
For these reasons, the predetermined rotational speed for disabling the electrical tool equipped with such a conventional safety device is not necessarily set to a most desirable value, while the predetermined rotational speed could hardly be changed.
According to one conventional safety device of this sort, an initial tension of the elastic member for drawing the cutter towards the rotary member is adjustable by changing the attaching position of the elastic member. However, in this conventional arrangement, since only the initial tension can be adjusted, a critical value of the rotational speed, above which the cutter protrudes, drastically varies when the attaching position of the elastic member is changed. Therefore, fine adjustment of the critical value of the rotational speed has been difficult hitherto.